Acylphosphonates: P-C Bond Cleavage of Dialkyl Acylphosphonates by Means of Amines. Substituent and Solvent Effects for Acylation of Amines

Article abstract of DOI:10.1021/jo01309a019

Studies on the benzoylation of amines of dialkyl benzoylphosphonates (1A-F) were described in detail. Stoichiometric reactions of diethyl benzoylphosphonate (1B) with a variety of amines (2a-i) gave amides as the main products along with diethyl phosphonate (4B) and α-(phosphoryloxy)benzyl phosphonate (5B). The yields of amides increased with the ratio of 1B/2a-i. The use of hindered dialkyl benzoylphosphonates resulted in high yields of amides while the reaction rates decreased markedly. The benzoylations of n-propylamine (2d) with 1B in various solvents having dielectric constants of 1.9-36.7 were conducted. The yields of N-n-propylbenzamide (3d) and 5B were surprisingly almost constant. However, the reaction rates varied as follows. In nonpolar solvents such as n-hexane and cyclohexane benzoylation was remarkably rapid while the benzoylation in methylene chloride was much slower than that in other solvents used. Compound 1B underwent smooth reaction with aliphatic amines but did not react with aromatic amines under the same conditions. Selective N-benzoylation of the bifunctional amine, ethanolamine, was achieved by means of diisopropyl benzoylphosphonate (1C) in tetrahydrofuran. The use of the hindered dialkyl benzoylphosphonate 1C resulted in poorer yields of amides in the case of the reaction with a hindered amine such as diethylamine, but higher yields of amides in the case of primary amines. Addition of triethylamine and 4-(dimethylamino)pyridine (DMAP) slightly accelerated the benzoylation, but yields of amides were similar to those in the absence of the catalysts. Optimum conditions for high yields of amides were proposed for the practical use of dialkyl benzoylphosphonates as the acylating agents in the present reaction.